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Abstract:

A novel electro-optical sensor for the wideband and normalized
translation of the two-dimensional position of a light beam transverse to
its traveling direction into electrical position signals. Incident on the
sensor is the light beam 122 which is divided with a beamsplitter 121
into a transmitted beam 123 and a reflected beam 124 which both have
similar transverse motion behavior as the incident beam. From each of
these divided beams the position is determined one-dimensionally with an
one-dimensional optical position sensor, 125, 126. The one-dimensional
position determination is done by dissecting each divided beam into two
beams using a partitioning element. The outputted dissected beams have a
power distribution that depends on the position of the divided beam
relative to the partitioning element. Each beam is optically coupled to a
photo detector which translates its power into an electric current. In
each circuit 127, 128, the two photo detectors are reverse biased and
electrically connected in series. The node connecting them is a current
type position output. The sum current through the two photo detectors is
determined using two current mirrors. The first mirrors the current
through one photo detector, the second mirrors the output current of the
first current mirror plus the current through the second photo detector
and outputs a current type sum signal. Each position signal is normalized
with respect to its sum signal using an analog-to-digital converter, 129,
130, with the sum signal coupled to the reference input and the position
signal coupled to the regular signal input.

Claims:

1. A method for sensing the position of an incident optical beam in a
plane perpendicular to its traveling direction and converting the
position to an electrical signal, the method comprising: a) splitting
said incident optical beam into a first separate optical beam having
similar deflection behavior in a plane perpendicular to its traveling
directions as the deflection of said incident optical beam in a plane
perpendicular to its traveling direction and a second separate optical
beam having similar deflection behavior in a plane perpendicular to its
traveling directions as the deflection of said incident optical beam in a
plane perpendicular to its traveling direction using a beamsplitter; b)
converting the position of said first separate optical beam into a first
electrical signal using a first optical position sensor; c) converting
the position of said second separate optical beam into a second
electrical signal using a second optical position sensor;

2. A two-dimensional optical position sensor for sensing the
two-dimensional position of an incident optical beam in a plane
perpendicular to its traveling direction and conversion of the
two-dimensional position to an electrical signal, the sensor comprising:
a) an optical beam splitter having at least one optical input and two
optical outputs, capable of receiving the incident optical beam on an
optical input, dividing it into a transmitted beam and a reflected beam,
outputting the transmitted beam on a first optical output, and outputting
the reflected beam on a second optical output; b) a first optical
position sensor optically coupled to the first optical output of the
beamsplitter, capable of sensing the position of said transmitted beam in
at least one dimension, converting this position to a first electrical
signal, and outputting the first electrical signal; c) a second optical
position sensor coupled to the second optical output of the beamsplitter,
capable of sensing the position of said reflected beam in at least one
dimension, converting this position to a second electrical signal, and
outputting the second electrical signal;

3. The sensor of claim 2 wherein the optical beam splitter is a cube
beamsplitter.

4. The sensor of claim 2 wherein the optical beam splitter is a plate
beamsplitter.

5. The sensor of claim 2 wherein the optical beam splitter is a
polarizing beamsplitter.

6. The sensor of claim 2 wherein at least one optical position sensor is
a two segment photo diode.

7. The sensor of claim 2 wherein at least one optical position sensor is
a quad segment photo diode.

8. The sensor of claim 2 wherein at least one optical position sensor is
a low noise optical position sensor as described in patent US005880461A.

9. The sensor of claim 2 wherein at least one of the optical position
sensors is mounted on a linear stage.

10. The sensor of claim 2 wherein at least one of the optical position
sensors is mounted on a rotary stage.

11. The sensor of claim 2 further comprising: At least one focusing
element, located before the optical input of the beamsplitter to
concentrate the said reflective and said transmitted beams on the photo
detectors of said optical position sensors.

12. The sensor of claim 2 further comprising: At least one focusing
element, located between an optical output of the beamsplitter and a
coupled optical position sensor to concentrate the beam on the photo
detectors of said optical position sensor.

13. An optical position sensor for sensing the position of an optical
beam and conversion of the position to an electrical signal, the sensor
comprising: a) an optical partitioning element capable of receiving the
optical beam and having at least two optical outputs; b) an optical
sensor coupled to the optical partitioning element, the optical sensor
having: a first output terminal; a second output terminal; a third output
terminal; a fourth output terminal; a first photo detector which produces
an electrical signal in response to a light input, coupled between the
first and second output terminals, the first photo detector element being
optically coupled to the first output of the optical partitioning
element, and; a second photodetector which produces an electrical signal
in response to a light input, coupled between the third and fourth output
terminals, the second photo detector element being optically coupled to
the second output of the optical partitioning element.

14. The sensor of claim 13 wherein the partitioning element is a prism.

15. The sensor of claim 14 wherein the prism has reflective legs.

16. The sensor of claim 13 wherein the first and second photo detectors
are PiN type photodiodes.

17. The sensor of claim 13 wherein the first and second photo detectors
are Indium Gallium Arsenide.

18. The sensor of claim 13 wherein the first and second photo detectors
are avalanche type photodiodes.

19. The sensor of claim 13 wherein the first and second photo detectors
are photo-transistors.

21. The sensor of claim 13 wherein the light beam deflection on the light
input represents bits of data.

22. The sensor of claim 13 wherein the first and second photo detectors
are tilted with respect to their incident beams and reflect the incident
light on a light absorber.

23. The sensor of claim 13 wherein the first and second photo detectors
are tilted with respect to their incident beams and reflect the incident
light on a light reflector that reflects the incident light back on the
photodetector.

24. The sensor of claim 13 further comprising: two optical focusing
elements which are located between the outputs of the partitioning
elements and their coupled photo detectors as to concentrate each beam on
its respective photo detector.

25. The sensor of claim 13 wherein the outputs of the detectors are
coupled to amplifier electronics.

26. The sensor of claim 13 wherein the optical sensor further includes a
first voltage source coupled in series with the first photo detector and
a second voltage source coupled in series with the second photo detector.

27. The sensor of claim 26 wherein the first and second voltage sources
are transformer isolated DC power supplies.

28. The sensor of claim 13 wherein the cathode of the first photodetector
is coupled to a positive bias voltage, the anode of the first photo
detector is connected to the cathode of the second photo detector, the
anode of the second photo detector is connected to a negative bias
voltage, and an output node coupled to the anode of the first photo
detector and the cathode of the second photo detector.

29. The sensor of claim 28 wherein the output node coupled to the anode
of the first photo detector and the cathode of the second photo detector
is coupled to amplifier electronics.

30. The sensor of claim 28 wherein the output node coupled to the anode
of the first photo detector and the cathode of the second photo detector
is coupled to the first terminal of a resistor which is coupled to ground
with a second terminal.

31. The sensor of claim 13 coupled to an electronic circuit comprising:
a) two NPN type transistors; b) a positive bias voltage source; c) a
negative bias voltage source; d) a first circuit output node; wherein the
cathode of the first photo detector is coupled to a positive bias
voltage, the anode of the first photo detector is coupled to the base of
a first NPN type transistor, the collector of the first NPN type
transistor is connected to the positive bias voltage, the emitter of the
first NPN type transistor is connected to the cathode of the second photo
detector, the cathode of the second photo detector is connected to the
collector of a second NPN type transistor, the anode of the second photo
detector is connected to the base of the second NPN type transistor, the
emitter of the second NPN type transistor is connected to a negative bias
voltage, and a first circuit output node coupled to the emitter of the
first NPN type transistor and the cathode of the second photo detector
and the collector of the second NPN type transistor.

32. The apparatus of claim 31 wherein the circuit output node is coupled
to amplifier electronics.

33. The apparatus of claim 31 further comprising: a resistor having two
terminals; wherein a first resistor terminal is connected to the circuit
output node, and a second resistor terminal is connected to ground.

34. The sensor of claim 13 coupled to an electronic circuit comprising:
a) two PNP type transistors; b) a positive bias voltage source; c) a
negative bias voltage source; d) a first circuit output node; wherein the
emitter of a first PNP type transistor is connected to a positive bias
voltage, the cathode of the first photo detector is coupled to the base
of the first PNP type transistor, the anode of the first photo detector
is coupled to the collector of the first PNP type transistor, the emitter
of a second PNP type transistor is connected to the collector of the
first PNP type transistor, the cathode of the second photo detector is
coupled to the base of the second PNP type transistor, the anode of the
second photo detector is coupled to the collector of the second PNP type
transistor, and the collector of the second PNP type transistor is
coupled to the negative bias voltage, and a first circuit output node
coupled to the collector of the first PNP type transistor and the anode
of the first photo detector and the emitter of the second PNP type
transistor.

35. The apparatus of claim 34 wherein the circuit output node is coupled
to amplifier electronics.

36. The apparatus of claim 34 further comprising: a resistor having two
terminals; wherein a first resistor terminal is connected to the circuit
output node, and a second resistor terminal is connected to ground.

37. An optoelectronic system for converting a differential light signal
incident on two photodiodes to an electric position and sum signal, the
system comprising: a) a NPN current mirror having an input, a common, and
at least one output; b) a PNP current mirror having an input, a common,
and at least one output; c) a first photodiode having a first anode and a
first cathode; d) a second photodiode having a second anode and a second
cathode; e) a first input node; f) a second input node; g) a first output
node; h) a second output node; wherein the common of the PNP current
mirror is coupled to the first input node, an output of the PNP current
mirror is coupled to the first output node, the input of the PNP current
mirror is coupled to the first cathode, the first anode is coupled to the
second cathode, the second anode is coupled to the NPN current mirror
input, an output of the NPN current mirror is coupled to the input of the
PNP current mirror, the common of the NPN current mirror is coupled to
the second input node, and the second output node is coupled to the first
anode and the second cathode, whereby the first output node provides the
electric sum signal and the second output node provides the electric
position signal.

38. The system of claim 37 further including a positive voltage source
coupled in series between ground and said first input node, and a
negative voltage source coupled in series between ground and said second
input node.

39. The system of claim 38 wherein the positive and negative voltage
sources are transformer isolated DC power supplies.

40. The system of claim 37 wherein at least one of the current mirrors is
a Wilson type current mirror.

41. The system of claim 37 wherein at least one of the current mirrors
has an output current in fixed proportion to the input current.

42. A system for normalizing a first electrical analog signal with
respect to a second electrical analog signal and converting the result to
a digital signal, the system comprising: an analog-to-digital converter
having an analog signal input, an analog reference input, and a digital
signal output; wherein said first electrical analog signal is input in
said analog signal input, and said second electrical analog signal is
input in said analog reference input, whereby said result is presented at
said digital output.

43. The system of claim 42 further comprising: a digital-to-analog
converter having a digital signal input and an analog signal output;
wherein said digital signal output is coupled to said digital signal
input, whereby said result is presented at said analog signal output.

44. The system of claim 42 wherein the analog-to-digital converter has a
parallel architecture.

45. The system of claim 42 wherein the analog-to-digital converter is a
Flash type analog-to-digital converter.

46. The system of claim 42 wherein the analog-to-digital converter is a
pipeline analog-to-digital converter.

Description:

FIELD OF INVENTION

[0001] This invention relates to an electro-optic optical position sensor.
More particularly, the invention relates to a method and system for
receiving and converting optical signals to a position and a sum signal
with high bandwidth and high signal to noise ratio, and a method for the
high bandwidth normalization of one such signal with respect to another
such signal.

BACKGROUND OF THE INVENTION

[0002] Optical position sensors are used to monitor the location of an
optical spot that is incident upon the active area surface of a device.
They are commercially available in a one-dimensional and a
two-dimensional version, which are also commonly called a Position
Sensitive Detector (PSD) or a Position Sensitive Photo Detector (PSPD).

[0003] Two types of monolithic position sensitive photo detectors are
widely available for measurements in one dimension. The lateral effect
detector incorporates an electric resistive layer over the active surface
area of a single photo diode, with electrical contacts at either end of
the layer. This type of detector is useful for measuring the centroid of
an optical spot that may move across the entire photosensitive area. A
second type, called the bi-cell, is sensitive to displacements that are
small compared to the size of the optical spot, and commonly is used to
monitor perturbations of a probe beam caused by mechanical vibrations or
optical misalignment. The bi-cell, as the name already indicates, is
composed out of two cells, I, 1 and, II, 2, that are adjacent to each
other and have a small gap 3 between them, See FIG. 1A. The surface of
each cell represents a photodiode. The photodiodes are fabricated on a
monolithic chip and share the cathode 4 and have an individual anode, 5
and 6, see FIG. 1B. In a position detector system, see FIG. 1C, their
anodes are typically connected to transimpedance amplifiers 7 or so
called I-V converters that convert the current signals from the
photodiodes, II and III that run through wire 5 and 6, into
inverted voltage signals -UI and -UII at 10 and 11. A
differential/sum amplifier 12 subtracts -UI from -UII and
outputs the result (UI-UII) on 13. A summing amplifier 14 sums
-UI and -UII, inverts this, and outputs the result
(UI+UII) on 15. The I-V converters, the differential/sum
amplifier and the summing amplifier are typically constructed using
operational amplifiers (opamps).

[0004] The difference signal (UI-UII), 13, is connected to the
divider 16 at the signal input 17 pin. The sum signal (UI+UII),
15, is connected to the denominator input, 18, of the divider. The output
of the divider, 19, represents the one-dimensional position of the
optical spot normalized with respect to the detection range and is
mathematically represented by
PID=(UI-UII)/(UI+UII).

[0005] Theoretically the difference signal (UI-UII) corresponds
with the location of the optical spot on the active areas, and the sum
signal (UI+UII) corresponds with the total light intensity on
the active areas. In order to get a position value PID that depends
on the light spot size and is independent of the total light intensity
the normalization is performed, which is done by dividing the difference
signal with the sum signal. This links the output voltage directly to the
size of the optical spot making it sensitive to small position
perturbations of the beam and reduces noise that is induced by light
intensity variations.

[0006] The two-dimensional monolithic PSDs are also widely available in
two types, which are derived from the one-dimensional version. There is a
duo lateral effect detector (L. Lindholm, PCT/SE2001/001235) which is
useful for measuring the two-dimensional position of the spot centroid
across the entire photosensitive area. And there is a quad-cell, or so
called quadrant photodiode or four-segment photodiode that is sensitive
to displacements that are small compared to the size of the optical spot
and is frequently used in Atomic Force Microscopy (AFM), Friction Force
Microscopy (FFM), Scanning Probe Data Storage, optical levers,
autocollimators, optical beam profiling, laser beam position sensing, and
other alignment applications. Like the name indicates, it is composed out
of four active cells, denoted A, B, C, and D, which are ordered in a
2×2 array and are separated by a small gap, see FIG. 1D. Like the
bi-cell, each active cell of the quad-cell represents a photodiode which
are fabricated together on a monolithic chip and have a shared cathode,
21, and a individual anode, 22, 23, 24, 25, see FIG. 1E. Atypical
two-dimensional position detector system is constructed using opamps and
contains four I-V converters, 26, two sum/difference amplifiers, 27, one
summing amplifier, 28, and two dividers, 29, See FIG. 1F. There are two
electrical outputs, 30 and 31, one for the normalized vertical position
signal, PV=(UA+UB-(UC+UD))/(UA+UB+U.su-
b.C+UD), and one for the normalized horizontal position signal,
PH=(UB+UD-(UA+UC))/(UA+UB+UC+U.su-
b.D).

[0007] Like with the bi-cell, the normalization is performed by dividing
the difference signal with the sum signal in order to get the normalized
vertical position value PV respectively a normalized horizontal
position value PH. The two difference signals,
(UA+UB)-(UC+UD) and
(UB+UD)-(UA+UC), are analogs of the relative
intensity difference of the light sensed by opposing pairs of the
photodiode quadrant elements. The sum signal,
(UA+UB+UC+UD), is the analog of the intensity sensed
by all four quadrant elements together.

[0008] The conventional bi- or quad-sensors require I-V and
sum/subtraction stages in order to retrieve the position signals from the
photo sensitive segments. The I-V stage transforms the current coming out
of the photo segments into a corresponding voltage which then by means of
the sum and difference amplifiers is transformed into a voltage type
position signal. This means that the signal traverses two sequential
opamp stages before the position signal can be normalized, see FIGS. 1C,
and 1F.

[0009] The use of opamps has serious downsides. Wideband I-V converters
tend to be unstable (pag. 29, Mark Johnson. Photodetection and
Measurement, Mc Graw-Hill, ISBN 0-07-140944-0) and add unwanted
oscillatory signals to the position signal that decreases the maximum
obtainable position resolution. Furthermore, conventional
frequency-compensated opamps use an internal RC combination to give a
dominant frequency pole at around 20 Hz. Above this frequency the gain
drops off at a rate of -20 dB/decade, reaching 0 dB (unity gain) at the
frequency corresponding to the Gain Bandwidth Product (GBW). The gain is
therefore an approximately inverse function of frequency over the useful
frequency range. The gain at any frequency f is approximately GBW/f, and
the upper frequency limit or bandwidth of the transimpedance amplifier
is: flimit=(GBW/{flimit2πRlCp})1/2. This is
an approximate expression (pag. 29, Mark Johnson. Photodetection and
Measurement, Mc Graw-Hill, ISBN 0-07-140944-0) and should not be relied
on for exacting accuracy. Typically the limiting frequency is half the
value calculated from this expression. So, due to the use of opamps the
bandwidth of the position sensor is severely limited.

[0010] For a typical wideband photodiode transimpedance amplifier
consisting out of a 12 pF photodiode connected to an OPA657 opamp having
a GBW of 1.6 GHz and a 200 kOhm feedback resistor, the bandwidth is 10
MHz (Texas Instruments, OPA 657 Datasheet).

[0011] Also, the sum and difference amplifiers have a bandwidth that is
limited by the Gain Bandwidth Product (GBP) of the opamps used. (P.
Horowitz, The art of electronics, Cambridge University Press). The sum
and sum/subtraction stage also contains a feedback resistor, see FIG.
1.F, and when concatenated to the above wideband photodiode
transimpedance amplifier would reduce the bandwidth of the sensor system
even further, to values below 10 MHz.

[0012] The fastest optical position sensor reported is the sensor used by
Toshio Ando of Kanazawa University Japan (T. Ando et. al., Eur J Physiol,
DOI 10.1007/s00424-007-0406-0, Springer-Verlag, 2007) and is used in an
optical beam deflection detector for high speed atomic force microscopy
and nano visualization of biomolecular processes. The sensor is based on
a four-segmented 3 pF 40 MHz Si Pin photodiode and a custom-made fast
amplifier/signal conditioner having a bandwidth of about 20 MHz. Usually
in high speed microscopy the position signals are not normalized because
analog signal dividers are limited to about 10 MHz (Analog Devices,
AD734) and real-time digital division introduces unwanted signal delays.

[0013] Furthermore, the feedback resistors of the I-V stage and the
sum/subtraction stage are a source of thermal noise known as Johnson
noise. It is highly desirable to be able to measure the position of an
optical spot with the least electrical noise possible (J. D. Spear, low
noise optical position sensor, pat. US005880461A). Spear has invented a
separate-bi-cell photodetector that does not require the difference
amplifier and therefore reduces the Johnson noise of the system by the
elimination of that stage. His design however still requires the use of
the bandwidth limiting opamp I-V amplifier or so called
current-to-voltage amplifier.

[0014] In his design, two separate photo segments are connected in
parallel to the I-V converter doubling the capacitance on the input of
the I-V converter with respect to that of a single photodiode connected.
Because the bandwidth of the sum/difference amplifier is much larger than
that of the I-V converter the connection of the second photodiode to the
opamp about halves the bandwidth of the position sensor system. Also,
because still the opamp I-V converter is used the unwanted oscillatory
distortion is still present in the output signal in wideband applications
together with the Johnson noise that originates from the feedback
resistor.

[0015] Furthermore the distortion of the position signal due to light
intensity fluctuations cannot be normalized out of this system because
the sum signal cannot be distracted from it. Their design demands that
the two photodiodes have to be connected in parallel to each other, "with
the cathode of the one photodiode connected to the anode of the other
photodiode and the anode of the said one photodiode connected to the
cathode of the said other photodiode". This parallel connection leaves no
connection left for the independent determination of the currents running
through each photodiode so that that the sum signal cannot be retrieved.
Hence making it impossible for the position signal to be normalized.

[0016] So, for wideband high sensitivity position measurements of an
optical beam the conventional detector cells having the standard two
stage opamp circuitry in combination with an analog signal divider are
considered to be the best measuring device available.

[0017] In our inventive two-dimensional optical position sensor the
decomposition of the two-dimensional input signal into two times a
one-dimensional signal is performed optically so that the electrical
determination of the position and sum signals can be performed for each
dimension independently from the other dimension. This enormously
increases bandwidth and signal-to-noise ratio, and decreases cross
couplings, noise, and system complexity.

[0018] Our inventive light beam position sensor does not require I-V
amplifiers nor does it require additional sum and/or sum/subtraction
stages for the retrieval of one-dimensional or two-dimensional position
signals. Hence it is not limited in bandwidth due to these amplifiers and
interconnections, and does it not contain the thermal noise and
oscillatory distortions these elements produce.

[0019] Also does our inventive sensor permit the retrieval of signals from
the photo-segments for the construction of the sum signal. We have also
invented a wideband method to perform the summing required without
interfering with the position signals. Hence, a wideband means for the
sum signal is provided while keeping the position signals wideband also.

[0020] Additionally, to solve the normalization problem, we have invented
a wideband normalization method capable of normalizing signals up to the
conversion rate of the fastest analog-to-digital converters available,
which is nowadays about 2 GS/s for a single analog-to-digital converter.
Interleaved analog-to-digital converter systems would increase the
bandwidth of the normalization method even further. This normalization
method does not require any extra components at all when the position
signal is to be digitized in the application where it is used in. This
would result in an enormous system simplification, cost reduction, and
energy saving, as the outputs of conventional position sensors are almost
always digitized for recording or processing purposes.

[0021] With the conventional sensor there is a separation gap, 3, between
the active areas. In cases where a non-homogenous light spot is used
errors occur due to this gap. This error becomes larger when smaller
photodiode areas are used because the area width to gap width ratio
decreases. The gap makes it impossible to use small photodiode areas
which are faster in response due to their lower internal capacitance.

[0022] In our inventive sensor there is no separation gap necessary
between the segments so that any errors that are produced due to this
separation gap are not present. This opens the opportunity to focus the
optical beam on small photodiode areas without the gap error.

[0023] In the prior art sensors back reflection of light in the direction
of the incoming beam can result in problematic errors. This can be
prevented in our inventive detector. It reduces optical pollution--which
is about 40% of the incident light--in the application wherein the sensor
is used. In applications where coherent light is used pollution due to
interference effects of the incoming bundle with the back scattered
bundle are thus removable.

[0024] In the alignment of a conventional two-dimensional sensor the
sensor needs to be moved as a whole in the plane of the
position-detection with the result that by movement of the center
position of the first axis there is always some uncontrollable movement
of the center position of the second axis. This unwanted dis-alignment of
the second axis center then requires supplemental alignment which in turn
dis-aligns the first axis center again. So, the alignment of the
conventional sensor is an iterative process requiring multiple steps
which is time-consuming to do manually and is difficult to make automatic
due to the iteration process involved.

[0025] Our inventive sensor has an unexpected result. It permits the
horizontal and vertical center positions to be adjusted without
cross-couplings to each other so that the alignment of the axis can be
done in one step which can be quickly performed manually and would be
easily to implement automatically.

[0026] Thus a need exists for a one-dimensional and two-dimensional
optical position sensor which allows high bandwidth without sacrificing
signal to noise ratio unnecessary. There is further need for a high speed
optical position sensor that also outputs the total intensity (sum)
signal at high bandwidth. There is also a need for an optical position
sensor which allows electrical design flexibility in its output
properties for integration with other electronics. There is also a need
for the high and wide bandwidth normalization of a position signal with
respect to a sum (total intensity) signal.

[0027] Furthermore there is a need for an optical position sensor that;
has a low amount of back scattered light, that allows easy and direct
alignment without cross coupling among its axis, that has a low
temperature drift, and has no dead gap between the photo sensitive
segments.

SUMMARY OF THE INVENTION

[0028] These needs may be addressed by the present invention which is
embodied in a novel two-dimensional optical position sensor for reception
of the two-dimensional deflection of an optical beam and conversion of
that signal to a two-dimensional electrical wideband normalized position
output signal, see FIG. 8.

[0029] The sensor has a beam splitter 121 capable of dividing an input
beam 122 in two output beams, 123, 124, that have similar deflection
behavior as the input beam. For each of these output beams a
one-dimensional position component is measured by a wideband
one-dimensional optical position detector, 125, 126. The joint output
signal of the two one-dimension position sensors is a two-dimensional
position signal corresponding to the position of the optical beam
incident on the beamsplitter. For one-step alignment each one-dimensional
photodetector is mounted on a linear adjustable stage.

[0030] In each one-dimensional position detector a partitioning element
dissects an input beam in two output beams which each have a beam width
that depends on the spatial position of the input beam with respect to
the partitioning element. The first partition element output beam is
incident on a first optical sensor and has a first output terminal and a
second output terminal.

[0031] A first photo detector which produces an electrical signal in
response to a light input is coupled between the first and second output
terminals. The first photo detector is exposed to the first output of the
partitioning element. The second partition element output beam is
incident on a second optical sensor and has a third output terminal and a
forth output terminal. A second photo detector which produces an
electrical signal in response to a light input is coupled between the
third and forth output terminals.

[0032] By tilting the photodetector areas with respect to their input
beams a low back reflection coefficient is obtained and responsivity can
be increased. By using a symmetrical partitioning element the output has
low thermal drift. And by using a sharp partitioning elements no dead
gaps exists enabling the focusing of the light beam on small area
photodiodes without introducing a gap error.

[0033] In the circuits 127, 128, that retrieve the position and sum
signals from the one-dimensional optical position sensors, the
photodiodes from each detector are connected in series by connecting the
anode of one of the photodiodes to the cathode of the other photodiode.
The photodiodes are reverse biased by applying a positive bias voltage to
the free cathode and a negative bias voltage to the free anode. Due to
the series connection the node connecting the photodiodes together
provides a current output that corresponds with the beam position
relative to the center of the partitioning element. This output current
signal can be converted to a voltage signal by means of a load resistor.
By varying the load resistor the bandwidth and output signal range of the
sensor can be varied. Bandwidths of more than 1 GHz can be obtained
without amplification. With amplification higher bandwidths can be
obtained, up to 60 GHz at present.

[0034] The sum signal is derived using current mirrors. By means of a NPN
current mirror connected between the negative bias supply and the anode
of the negative reverse biased photodiode the current through this
photodiode is mirrored. The output of this NPN current mirror is
connected together with the cathode of the positive reverse biased
photodiode to the input of a PNP current mirror that is connected between
the cathode of the positive reverse biased photodiode and the positive
bias voltage. The output of the PNP current mirror presents the sum
current, which can also be converted to a voltage signal by means of a
load resistor. When, in comparison to the photodetectors, transistors
with a relatively low capacitance and differential resistance are used to
build the current mirrors, the sum signal output has about the same
bandwidth as the position signal output. To obtain larger signal values
without the expense of bandwidth, amplifiers can be used to amplify the
position and sum signals. Suitable amplifiers up to 60 GHz are available
at present. At the expense of bandwidth the output values can be
increased by amplifying the photo-currents with a transistor.

[0035] To normalize the position signal with respect to the sum signal an
analog-to-digital converter with a reference input is used 129, 130. To
apply the normalization method, the sum signal is connected to the
reference input of the analog-to-digital converter and the position
signal is connected to the regular input. The analog-to-digital converter
then outputs the normalized position signal digitally. By means of an
digital-to-analog converter this signal can be made analog if required.
Digital processing techniques can be applied on the digital normalized
position signal prior to the digital-to-analog conversion. Bandwidths of
2 GHz can be obtained using high speed flash type analog-to-digital
converters available at present. Using interleaved converters this
bandwidth can be increased even further.

[0036] It is to be understood that both the foregoing general description
and the foregoing detailed description are not limiting but are intended
to provide further explanation of the invention claimed. The accompanying
drawings, which are incorporated in and constitute part of this
specification, are included to illustrate and provide a further
understanding of the methods and systems of the invention. Together with
the description, the drawings serve to explain the principles of the
invention.

[0052]FIG. 3F is a top view of a fabrication assembly of FIG. 3A, where
the photodetector areas are tilted with respect to the incident beam and
reflect on a light absorber.

[0053] FIG. 3G is a top view of a fabrication assembly of FIG. 3A, where
the photodetector areas are tilted with respect to their incident beams
and reflect on a light reflective area that reflects back on the
photodetector area.

[0054]FIG. 4A shows the inventive circuit diagram where an output current
provides the sum signal.

[0056]FIG. 4c and FIG. 4D show the inventive circuit diagram where the
photo currents are amplified by means of an electron valve.

[0057] FIG. 5 shows different variations of FIG. 4 that are equivalent to
FIG. 4.

[0058]FIG. 6A shows the inventive circuit diagram where one output
current provides a measure of deflection and another output provides a
measure for the sum, which is derived from the photodetector currents by
means of two current mirrors.

[0059]FIG. 6B and FIG. 6C show the inventive circuit diagram of FIG. 6A,
where the photo-currents are amplified by means of an electron valve.

[0060] FIG. 7A shows the block diagram of the inventive normalization
method comprising an analog-to-digital converter (ADC) with a reference
input and a digital signal output that provides the normalized position
value.

[0061] FIG. 7B shows the block diagram of the inventive normalization
method comprising an analog-to-digital converter (ADC) with a reference
input and a digital-to-analog converter (DAC) that provides an analog
normalized position output value.

[0062]FIG. 7c shows the block diagram of the inventive normalization
method comprising an analog-to-digital converter (ADC) with a reference
input, a digital-signal-process (DSP), and a digital-to-analog converter
(DAC) that provides a processed analog normalized position output value.

[0063] FIG. 8 shows the block diagram of the inventive two dimensional
optical position sensor for reception of the two dimensional deflection
of an optical beam and conversion of that signal to a two-dimensional
electrical wideband normalized position output signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0064] While the present invention is capable of embodiment in various
forms, there is shown in the drawings and will hereinafter be described a
presently preferred embodiment with the understanding that the present
disclosure is to be considered as an exemplification of the invention,
and is not intended to limit the invention to the specific embodiment
illustrated.

[0065]FIG. 2A shows the perspective view of the inventive electro-optic
light beam position sensor that is capable of determining the
two-dimensional position transverse to the traveling direction of an
incoming light beam with respect to a center position. The position
transverse to the traveling direction of a light beam with respect to a
center position is also called the deflection of that light beam. It
works as follows.

[0066] The center of the incoming light beam 32 having a traveling
direction as indicated by 33 is incident on beam-splitter 34 at point 35
where at point 36 it hits the beam-splitters partial reflective surface
which is enclosed by 37. At the point where the incoming beam hits the
beam-splitters partial reflective surface one part of the light of the
incident beam is reflected in a direction dependent on the orientation of
the partial reflective surface with respect to the incoming beam while
the other part of the light of the incoming beam is not affected by the
partial reflective surface and is transmitted through the partial
reflective surface. Hence incoming beam 32 is split in two beams at point
36; a reflected light beam 38 having a traveling direction indicated by
39 that exits the beam-splitter at point 40, and a transmitted beam 41
having traveling direction 42 exiting the beam-splitter at point 43.

[0067] A deflection of the incoming beam 32 in the vertical direction
indicated by arrow 44 has as result that point 36 where the incoming beam
32 hits the partial reflective surface 37 is also deflected in the
direction as indicated by arrow 44. Due to the deflection of point 36
into direction 44 the reflected beam 38 is deflected in direction 45, and
the transmitted beam 41 is deflected in direction 46. Furthermore, a
deflection of the incoming beam 32 in the horizontal direction as
indicated by arrow 47 has as consequence that point 36 is also deflected,
and because point 36 moves over the partial reflective surface enclosed
by 37--and the reflective surface has an angle with respect to that
deflection direction--the deflection of the reflected beam 38 is in the
direction as indicated by arrow 48. The deflection of the transmitted
beam 41 as result to a deflection of the incoming beam 32 in the
horizontal direction 47 is in the direction of arrow 49. So, the
horizontal 48 and vertical 45 deflection of the reflected beam 38 and the
horizontal 49 and vertical 46 deflection of the transmitted beam 41 are
similar to the horizontal 47 and vertical 44 deflection of the incoming
beam 32.

[0068] There are now two beams available--the reflected beam 38 and the
transmitted beam 41--for the two-dimensional position determination of
the incoming beam 32 that both have similar deflection behavior as the
incoming beam 32. So, the two-dimensional deflection determination of the
incoming beam 32 can be decomposed in two times a one-dimensional
deflection detection where a first one-dimensional deflection detection
is performed on the reflected beam 38 and a second one-dimensional
deflection detection is performed on the transmitted beam 41. Now the
combination of one-dimensional measurements on the reflected and
transmitted beam is a two-dimensional measurement of the incoming beam,
whereby the axis of detection and the center position can be freely
chosen by the orientation of the two one-dimensional optical position
sensors.

[0069]FIG. 2B and FIG. 2C show the electro-optic setup of FIG. 2A as seen
from the top and illustrate how a beam deflection in the horizontal
direction would be determined by the sensor. In FIG. 2B the incoming beam
32 is about to deflect in direction 47, and FIG. 2C shows the situation
after beam 32 has performed the deflection.

[0070] In FIG. 2B, the incoming beam 32 has a horizontal width as
indicated by 62 and is about to move in the direction indicated by arrow
47. As a result the reflected beam 38 having a horizontal width as
indicated by 63 is about to move in the direction of arrow 48 and beam 41
having a width 66 is about to move in direction 49.

[0071] Sensor 50 is a one-dimensional deflection sensor containing a
positive area 52 and a negative area 53 which are separated by line 54.
The reflected beam center 38 hits the deflection sensor 50 at point 51. A
measure for the location of point 51 with respect to the separation line
54 of sensor 50 is the horizontal deflection value, H, to be obtained.

[0072] In FIG. 2B, the beam center 38 hits deflection sensor 50 at point
51 and is on the negative area 53 of it. In FIG. 2C, the beam center 38
hits sensor 50 at its positive area 52. So, the beam has moved from a
negative position value to a positive position value following the
figures from FIG. 2B to FIG. 2C.

[0073] Sensor 50 transforms the light power incident on its positive area
52 into electric signal H.sub.+ and transforms the light power incident
on its negative area 53 into electric signal H.sub.-. The amount of power
incident on the positive area 52 corresponds with the part of the beam
that is incident on area 52 and is indicated by 64. The amount of light
power incident on the negative area 52 in that respect is indicated by
65.

[0074] In FIG. 2B, where beam 38 is about to move into direction 48, the
power incident on the positive area 64 is less than the power incident on
the negative area 65. So, the beam power imbalance, where the power
incident on the positive area 52 is assigned a positive value and the
power incident on the negative area 53 is assigned a negative value, is
H.sub.+-H.sub.-, and has a negative value in FIG. 2B.

[0075] After the movement in direction 48, as is shown in FIG. 2C, the
beam power 64 incident on the positive area 52 is larger than the beam
power 65 incident on the negative area 53. Hence, the beam power
imbalance H.sub.+-H.sub.- in FIG. 2C has a positive value. The value
H.sub.+-H.sub.- is thus a measure for the horizontal deflection of the
reflected beam 38 in the positive--along with arrow 55--and negative--in
direction opposing arrow 55--direction. So, when electric signal His subtracted from the electric signal H.sub.+, a signal
H=(H.sub.+-H.sub.-) is obtained which corresponds to the horizontal
deflection of beam 38. When H is positive, it provides a value of how far
the center of the beam is away from the separation line 54 into the
positive area, and in the case it is negative provides a value of how far
the beam center is separated from the separation line 54 into the
negative area.

[0076] Furthermore signal H corresponds with the horizontal deflection 47
of the incident beam 32 and the horizontal deflection 49 of the
transmitted beam 41 and so provides a measure for the horizontal
deflection for each of these three beams.

[0077] When the reflected beam 38 would make a deflection in the vertical
direction 45 (see FIG. 2A), the deflection would be in parallel with the
separation line 54 that separates the positive 52 and negative 53 areas.
Because such deflection is in parallel with the separation, the part of
the beam power incident on the positive area 64 (see FIG. 2B) does not
change, and the part incident on the negative area 65 does not change
either. The so called power distribution--the part incident on 52 with
respect to the part incident on part on 53--remains unaltered. So, the
power imbalance, H.sub.+-H.sub.-, and so signal H remain unchanged. Hence
detector 50 does not detect deflections of the beam in the vertical
direction. This means that for the case a beam deflection is composed out
of a horizontal and a vertical part, only the horizontal part is sensed
by sensor 50. So, the output of sensor 50 is a value for the horizontal
component 47 of the deflection of the incoming beam 32.

[0078]FIG. 2D shows the electro-optic setup from FIG. 2A as seen from the
side and illustrates how a beam deflection in the vertical direction
would be determined. The incoming beam 32 having a vertical width as
indicated by 67 is incident on the beam-splitter 34. The vertical
direction is indicated by arrow 44. Vertical movement of the incoming
beam 32 would cause the transmitted beam 41 to move in the vertical
direction 46 also. The width of the transmitted beam 41 is indicated by
68 and is incident on a one-dimensional deflection sensor 56.

[0079] In principle sensor 56 works similarly as sensor 50 but its
orientation with respect to the incident beam differs. For sensor 56, the
line 60 separating its positive 58 and negative areas 59 is in parallel
with the horizontal direction 49. Hence, the detection direction of
sensor 56 is in the direction of the vertical axis 46 and is positive
along with arrow 61 and negative in the direction opposing arrow 61.

[0080] The beam power 69 incident on the positive area 58 is transformed
into a positive electric signal V.sub.+, the incoming beam power 70
incident on the negative area 59 is transformed into a negative electric
signal V.sub.-. The power imbalance that sensor 56 measures is
V.sub.+-V.sub.-, and corresponds with the vertical deflection V of beam
41. So, the vertical deflection V=(V.sub.+-V.sub.-). Similar in the way
like sensor 50 does not sense deflections in the vertical direction
sensor 56 does not sense deflections in the horizontal direction.
Combined with the fact that the deflection of the incoming beam 32 and
the transmitted beam 41 are similar the output of sensor 56 is a value
for the vertical component 44 of the deflection of the incoming beam 32.

[0081] In FIG. 2A. one one-dimensional value H of the deflection of the
incoming beam 32 is determined from the reflected beam 38 by sensor 50
and is output electrically 71, and one one-dimensional value V of the
deflection of the incoming beam 32 is determined from the transmitted
beam by sensor 56 and is output electrically 72. The combination 73 of
the two one-dimensional electric outputs 71 and 72 constitute a
two-dimensional electric signal (H, V) which corresponds with the
two-dimensional deflection of the incoming beam 32.

[0082] Furthermore, as an unexpected result, the inventive position sensor
also permits the horizontal and vertical center positions to be adjusted
without cross-couplings to each other. In the alignment of a prior art
sensor, the sensor needs to be moved as a whole in the plane of the
position-detection with the result that by movement of the center
position of the first axis there is always some uncontrollable movement
of the center position of the second axis. This unwanted dis-alignment of
the second axis center then requires supplemental alignment which in turn
dis-aligns the first axis center again. So the alignment of the prior art
sensor is an iterative process requiring multiple steps which is time
consuming to do manually, and is difficult to make automatic due to the
iteration process involved. Our inventive sensor does not require to be
moved as a whole over the plane of position detection for the alignment.
Internally the two one-dimensional position detectors--the two axis--can
be adjusted completely independent from each other by mounting each of
them on a linear adjustable stage. This has the result that movement of
the center position of the first axis does not alter the center position
of the second axis. Hence the alignment of the axis can be done in one
step which can be quickly performed manually, and would be easily to
implement automatically.

[0083] Also, the angles the axis have in the plane of position detection
are always perpendicular in the prior art sensor. They cannot be adjusted
with respect to each other. Usually when motion is measured from objects
by means of the optical lever method the eigenmodes of the motion are not
precisely perpendicular. This means that the affinity of the light beam
to be detected in the first dimension is not precisely perpendicular to
the affinity of the light-beam in the second dimension, so the desired
detection directions may have angles that are not 90 degrees to each
other. This means that the two dimensions of the position determination
can contain a cross-coupling with each other in the prior art sensor
which cannot be adjusted away internally and has to be done by an
external apparatus. In our embodiment the angles that the two dimensions
have with respect to each other, and with respect to the encasing of the
embodiment, can be adjusted fully over 360 degrees independently from
each other by mounting each them on a rotary stage. This enables the
alignment of the detection-axis with the non-perpendicular axis of the
two-dimensional signals. The rotary stage can furthermore be mounted to
the linear stage mentioned earlier or the detector together with the
linear stage can be mounted to the rotary stage.

[0084] For the one-dimensional position detection of the reflected and
transmitted beams several options are available. The conventional bi-cell
can be used, in combination with the opamp summing/subtraction stages, or
in combination with any other signal processing system, weather analog or
digital. Another option is to use the low-noise optical position sensor
of J. D. Spear described in patent US005880461A. Any position sensitive
photo sensor can be used. Both mentioned sensors would work fine but
still have a severely limited bandwidth. To overcome the limited
bandwidth problem we have invented a new one-dimensional optical beam
position sensor that enables the wideband detection of the position of an
optical beam with a bandwidth range from 0 Hz up to 60 GHz and more with
commonly available parts and can be made even higher when better
components become available.

[0085] The inventive one-dimensional optical position sensor consists out
of an optical part and an electrical part. The optical part of the
one-dimensional sensor (see FIG. 3) is designed symmetrically around the
detection center position in order to compensate for errors due to
thermal expansion of the sensor parts. This reduces the thermal drifts
which is visible on the sensor output signals of the prior art position
sensor. The electrical part is designed for simplicity, symmetry, signal
output compatibility, and design freedom, see FIG. 3D.

[0086]FIG. 3A illustrates the perspective view and FIG. 3B and FIG. 3C
illustrate the top view of how the optical part of the inventive
one-dimensional light beam position sensor works. The light beam with
center position 74 and width 75 is incident on a partitioning element 76.
This beam can be the transmitted beam 38 or the reflective beam 41 of the
setup depicted in FIG. 2A or the sensor can function as a standalone
one-dimensional position detector. In FIG. 3, the partitioning element 76
is a prism that has two reflective legs 77 and 78 that have a sharp
separation edge 79 between them. This has the result that the incoming
beam is separated in two beams where the distance between the center
position of the incident beam, 74, and the separation line of the
partitioning element, 79, determines what spatial part of the incident
beam is reflected in one direction, 80, and what other part of the
incident beam 74 is reflected in a other direction, 81.

[0087] Beam 80 has a width 82 and is incident on photodiode 83. Beam 81
has a width 84 and is incident on photodiode 85. (Beam 80 and 81 can also
be directed on the photodiodes by means of an optical fiber so that the
photodiodes can be placed closer to each other reducing the length of the
electrical wires. To do this, one or both beams are to be coupled into an
optical fiber, which can be done by means of an optical collimator, for
instance a convex lens. The beam exiting the fiber is then to be incident
on a photodiode, 83 or 85.) In the FIG. 3A and FIG. 3B, beam 80 and beam
81 both have equal width so they are in balance. FIG. 3C illustrates the
optical part of the inventive one-dimensional light beam position sensor
where the incident beam center is offset the separation line of the
partitioning element. From that illustration it is clear that due to this
offset, beam 80 is wider than beam 81. Hence, there is an imbalance among
the beams that corresponds with the offset between the incoming beam
center and the separation line of the partitioning element. The offset
between the beam center and the separation of the partitioning element is
the position of the beam relative to the partitioning element. Assigning
photodiode 83 to be the positive `+` segment and photodiode 85 to be the
negative `-` segment the signal from photodiode 83 and 85 correspond to
the signals from the conventional bi-cell position sensor areas I and II
as in FIG. 1A. Hence, the inventive one-dimensional light beam position
sensor determines the position of the incident beam 74 in the direction
of arrow 86.

[0088] The partitioning element 76 can be made out of any material, as
long as the legs are reflective. In case the material used does not have
reflective properties it can be made reflective by adding a reflective
coating to it. These methods are well known today. Thermal expansions are
compensated as long as the angles between the reflective legs and the
incident beam 74 are equal.

[0089] Also the angle between the legs does not have to be 90 degrees, so
the prism shape is not a necessity and other shapes would also suffice as
long as one spatial part of the beam is reflected in a different
direction than the other spatial part of the beam.

[0090] Using conventional techniques, the edge of the separation line 79
can be made nearly atomically sharp. This means that our inventive sensor
can be made without a separation gap, what is not possible with the
conventional bi-cell or quad-cell. Without a gap, no gap error exists,
and small area photodiodes can be used with equal precision as larger
area photodiodes. Smaller area photodiodes have a smaller internal
capacitance improving the bandwidth of the sensor. In case optical beams
that are too wide for the photodiode area to enclose are required, the
beam(s) can be focused on the photodiodes. This can be done by placing a
convex lens before the beamsplitter 34, between the beamsplitter 34 and
each one-dimensional optical sensor 50, 56, or between the partitioning
elements 76 and the photodiodes, 83 or 85. In cases a separation gap is
required, the sharp separation edge can be beveled to any value.

[0091] For the one-dimensional position detection of the horizontal
motion, as was illustrated in FIG. 2A, the separation line 54 of the
one-dimensional position sensor 50 corresponds with line 79 of a one
dimensional position detector and the detection direction 55 corresponds
with arrow 86. Signal H.sub.+ then comes from photodiode 83 and signal
H.sub.- from photodiode 85. For the vertical position detection another
one-dimensional position detector is used, sensor 56. The separation line
60 of this one-dimensional sensor corresponds with line 79 of the
inventive one-dimensional optical position sensor. Detection direction 61
then corresponds with arrow 86 and signal V.sub.+ and V.sub.- correspond
with the signals form photodiodes 83 and 85 respectively.

[0092] For the mechanical mount of the photodiodes 83 and 85 the plane of
the photodiodes can be tilted with respect to the incoming light bundle
to reduce back reflection of light in the direction of the incoming beam
by reflecting it in another direction. This reduces optical
pollution--which is about 40% of the incident light--in the application
wherein the inventive sensor is used. In applications where coherent
light is used pollution due to interference effects of the incoming
bundle with the back scattered bundle are thus removable. To do this, the
light bundle reflected from the photodiode is be made incident on a light
absorber, with the result that the light pollution is almost completely
eliminated, see FIG. 3F.

[0093] The back-reflection can also be used to enhance the responsivity of
the photodiode. The reflection from the photodiode is then incident on a
reflector so that the light is reflected back on the photodiode again,
see FIG. 3G. This would increase the amount of light incident on the
sensor by about 36% and thereby reduces the polluting light exiting the
inventive sensor with the same amount.

[0094] The enormous advantage of our inventive design is that the cathodes
87, 89 and the anode 88, 90 of each photodiode 83, 85 are free (see FIG.
3D) and are not joint as in the monolithic bi-cell or quad-cell case (see
FIGS. 1B and 1E). This advantage enables the development of simple,
symmetrical, and few component DC wideband electronics that construct the
position and sum signals from the photodiode currents, see FIGS. 4, 5,
and 6.

[0095] Furthermore it also can be made electrically compatible with the
bi-cell pin-out by connecting the cathodes together, or electrically
compatible with the low noise sensor of J. D. Spear as described in
patent US005880461A, by connecting the anode of the first photodiode to
the cathode of the second photodiode and connecting the cathode of said
first photodiode to the anode of said second photodiode.

[0096] The simplest circuit that obtains the one-dimensional position
signal and has a bandwidth ranging from DC up to very high frequencies
(>1 GHz) is illustrated in FIG. 4A. Here photodiode 83 is placed in
series with photodiode 85 with the anode 88 of photodiode 83 connected
with the cathode 89 of photodiode 85 at node 91. To decrease the capacity
of the photodiodes, a positive bias voltage 92 is applied to the cathode
87 of photodiode 83 and a negative bias voltage 93 is applied to the
anode 90 of photodiode 85. So both photodiodes operate in reverse bias
mode and are photo conductive. According to the Kirchhoff current law
node 91 acts as a current source and sink. The current output of node 91
is the photo-current through photodiode 83, I.sub.+, minus the
photo-current through photodiode 85, I.sub.-. So the current signal
output by node 91 is IP=I.sub.+-I.sub.- and corresponds with the
one-dimensional position of the optical beam incident on the
one-dimensional optical position sensor.

[0097] In case a voltage signal is required, a load resistor 94 can be
placed between node 91 and ground, see FIG. 4B. The load resistor with
value R1 transforms the current output into a voltage output where
the output voltage is UP=Rl×IP.

[0098] The load resistor also functions to quickly discharge the internal
capacitance of the photodiodes after the detection of a change in the
position of the light beam. However, at high frequencies above 1 GHz,
Johnson noise from the load resistor is predominant. Johnson noise is
inversely related to a resistor's ohmic value by the equation
I2noise=4KT/R where K is the Boltzmann constant, T is the
temperature and R is the resistor value. The lower the resistor value,
the larger is the Johnson noise current.

[0099] A larger resistor value creates a larger output signal voltage
value. A lower resistor value creates a higher bandwidth by reducing the
RC time constant which is governed by the load resistor value and the
internal capacitance of the photodiodes. Resulting in a compromise
between signal amplitude, bandwidth, and noise.

[0100] In wideband applications the cable connecting the output node 91
with the device that records the position information might be regarded
long with respect to the wavelength of the higher detectable frequencies.
For frequencies of 100 MHz, a cable of 50 cm is already regarded as long.
(For 1 GHz this is 5 cm.) Long lines are subject to internal signal
reflections that back-scatter in case the load applied at the end of the
cable is not equal to the characteristic impedance of the cable itself.
The signal reflections distort the original signal. Typical wideband
(coax, band, stripline) cables are designed that have a characteristic
impedance of 50 Ohms, 75 Ohms, or 300 Ohms. Other values also exist and
can also be used. A matching load resistor or load network eliminates
cable reflections.

[0101] For a 635 nm (red) laser with a power of 1 mW incident on the
one-dimensional sensor containing two photodiodes of 3 pF capacitance
each and a responsivity of 0.35 A/W at 635 nm. The current IP is
0.35 mA when the position of the light beam is utterly positive, and is
-0.35 mA when the light beam is positioned utterly negative. So the
top-to-top signal current variation is 0.70 mA, which generates a
top-to-top voltage of 210 mV over a 300 Ohms load resistor when placed at
the end of the `long` cable. The RC time of the photodiodes in
combination with the load resistor is 1.8 nano seconds, 241 MHz. The 210
mV top top signal can be used directly as input signal on a high speed
analog to digital converter.

[0102] In case larger signals are needed, a more powerful light beam can
be used. Using a 5 mW laser the top top output signal range changes to
about 1 Volts. An avalanche photodiode can also provide a larger signal,
typically 75 times larger than when a pin photodiode is used. In case
higher bandwidths are required, the load resistor can be reduced. For the
example given above, a 50 Ohm load resistor alters the RC time to 0.3 nS,
1450 MHz.

[0103] In case larger signal values are required without altering optical
beam power or output load resistor value, the signals can be amplified by
means of an amplifier. Suitable wideband amplifiers up to 60 GHz are
commercially available at present and the design of these amplifiers is
current art.

[0104] In case larger signals are required without using an avalanche
diode but with a low load resistor, the photo-current can also be
amplified by means of an electron valve, such as a transistor, see FIGS.
4C and 4D.

[0105] In FIG. 4c, the photo-current of photodiode 83 is led into the base
of an NPN type transistor 95 by connecting the anode 88 of photodiode 83
to it. The collector of transistor 95 is connected to the cathode of the
photodiode 87, which again is connected to the positive bias voltage
supply 92. Out of the emitter of transistor 95 now flows a current which
is the transistor current amplification factor+1, (β+1), times the
photo-current I.sub.+.

[0106] The photo-current from photo-diode 85 is led into the base of
another NPN type transistor, 96, by connecting its anode 90 to the base
of that transistor. The collector is connected to the cathode of the
photodiode, 89, and the emitter is connected to the negative bias voltage
supply 93. The emitter of transistor 95 is connected to the output node
91 and sources a current of (β+1)I.sub.+ to it. The collector of
transistor 96 and the cathode of photodiode 85 are also connected to the
output node 91 and together sink a current of (β+1)I.sub.- from it.
Hence, the output current of node 91 is
IP=(β+1)(I.sub.+-I.sub.-). The NPN transistors amplify the
current into the base by about 100 times, and depends on the specific
transistor used. So depending on the required load resistor, specified
output voltage range, bandwidth, and photo-current range, a pair of
transistors can be selected (or designed) that amplify the photo-currents
to the desired value.

[0107] The voltage output in case of a 50 Ohms load resistor in
combination with a 1 mW laser and transistors with a β of 100 times,
the output would now be about 3.5 volts top-to-top. This type of voltage
sizes can be very well measured by an analog-to-digital converter (ADC).
Further, transistors 95 and 96 can be chosen a matched pair in order to
enhance the identical behavior among the current sourcing side and the
current sinking side.

[0108] Instead of using NPN type transistors for the current
amplification, PNP type transistors can also be used, See FIG. 4D. In
FIG. 4D the cathode 87 of photodiode 83 is connected to the base of PNP
transistor 97. The emitter of transistor 97 is connected to the positive
bias voltage supply 92. The anode 88 is connected to the collector of
transistor 97 and together form a (β+1)I.sub.+ current source and
are connected to the output node 91. The cathode 89 of photodiode 85 is
connected to the base of the other PNP transistor 98. The anode 90 is
connected to the collector of transistor 98 and together are connected to
the negative bias voltage supply 93. The emitter of transistor 98 forms a
(β+1)I.sub.- current sink and is connected to the output node 91.
Hence, for the circuit depicted in FIG. 4D output node 91 is a
IP=(β+1)(I.sub.+-I.sub.-) current source.

[0109] The amplification factor of PNP type transistors typically is lower
than that of the NPN type transistors. The amplification of PNP
transistors is about 50 times, and also depends on the specific
transistor used. So, depending on the required load resistor, specified
output voltage range, and photo-current range, a pair of transistors can
be selected that amplify the photo-currents to the desired value.
Further, transistors 97 and 98 can be chosen a matched pair in order to
enhance the identical behavior among the current sourcing side and the
current sinking side.

[0110] On each side of the circuit loop in FIGS. 4A, 4B, 4C, and 4D, the
circuit between 91 and the positive bias voltage supply (the photodiode
with or without electron valve), and between node 91 and the negative
voltage power supply (the other photodiode with or without electron
valve) can be transposed in position (without rotation) with their
respective bias voltage power supplies without affecting the basic
electrical loop function. This is illustrated in FIGS. 5A, 5B, 5C and 5D
for the NPN amplified case and similarly is applicable to the other
circuits of FIG. 4.

[0111] Besides the large bandwidth, few component construction, and wide
output compatibility shaping ability (shaping among: noise, bandwidth,
output range, and output load), one other advantage of our inventive
electrical design is that the sum signals can be easily distracted from
the electrical system while affecting the response of the position
signals hardly. This is done by using current mirrors.

[0112] For the simplest design, originally illustrated in FIG. 4A, the sum
stage addition is illustrated in FIG. 6A.

[0113] The current running through the negative photo-segment 85 (I.sub.-)
is mirrored by a NPN transistor current mirror 99, which has a current
input 100, a current output 101, and a common which is connected to the
negative voltage supply. The current running through photodiode 85 flows
through wire 100, and current mirror 99 outputs a mirror of this current
through wire 101.

[0114] The current through the positive photodiode 83 (I.sub.+) runs
through wire 102. Now, due to the connection of wire 101 to wire 102 at
node 103, the current through wire 104 is the sum of the positive
photodiode current I.sub.+ plus the mirrored negative photodiode current
I.sub.-, hence through wire 104 runs the sum current,
IS=(I.sub.++I.sub.-), corresponding to the total light intensity
incident on both photodiodes 83 and 85.

[0115] Current mirror 105 is a PNP transistor current mirror which haves
an input 104, an output 106, and a common which is connected to the
positive voltage supply. The PNP current mirror 105 creates a mirror of
the sum current that runs through wire 104 and outputs this mirror at
wire 106 which functions as source current output terminal for the sum
signal Is.

[0116] The differential resistance of the current mirror inputs--the
resistance between wire 104 and the positive bias voltage line and
between wire 100 and the negative bias voltage line-originates from the
PN junctions of the transistors and is usually much lower than 50 Ohms.
Hence, the behavior of the system as depicted in FIG. 4A is hardly
affected by the addition of the current mirrors as shown in FIG. 6A. The
sum current output is also wideband due to the small collector to emitter
capacitance's of the transistors.

[0117] Furthermore, the sum current Is output 106 can be transformed
into a voltage output Us in ways similar to the transformation of
the position current output Ip into a voltage Up is performed.
Also can it be amplified by means of an amplifier if a larger signal
range is required.

[0118] The distraction of the sum signal from the circuits shown in FIGS.
4B, and 4C is in method similar to that of FIG. 6A, see FIGS. 6B, and 6C.
In order to keep the sum output current on equal footing with the
position output current the current through wire 107, (β+1)I.sub.-,
is mirrored. This is done by current mirror 108 which sinks
(β+1)I.sub.- at wire 109. Through wire 110 runs (β+1)I.sub.+.
Wire 109 is connected to wire 110 with the result that the sum current,
(β+1)(I.sub.++I.sub.-)=Is, runs through wire 111. This sum
current is mirrored by current mirror 112 which outputs this at wire 113.
Wire 113 is a current source output for the sum current Is and has
about the same bandwidth as the position signal due to the small
collector to emitter capacitance's of the transistors. The output current
can also be transformed into a voltage using a resistor to ground or an
I-V converter if desired.

[0119] For the current mirrors, Wilson type current mirrors can also be
used. Further, all the current or voltage signals can be amplified
internal or external to the inventive systems. Also active I-V or V-I
converters can be used for any current to/from voltage transformations
and (anti-aliasing) filters can be applied for bandwidth matching
purposes or noise optimization. Furthermore RF design techniques can be
applied for achieving bandwidths well into the radio frequency domain.
These techniques are well known in the art and are used widely in
integrated circuit technology, analog signal processing, digital signal
processing, broadband systems, (optical) communication systems, and data
transmission applications.

[0120] Having the position signal and the sum signal available, the
normalized position signal can be obtained. The prior art method is to
use an analog divider to divide the position signal with the sum signal.
As discussed earlier this method is usable up to about 10 MHz. Digital
normalization is also a conventional option but has the downside that
there is a large input to output delay due to the digital calculation
time involved.

[0121] Our inventive high speed normalization system uses an
analog-to-digital converter (ADC) that has a signal reference input 114,
a signal input 115, and a digital output 116, see FIG. 7A. In such an
analog-to-digital converter the reference scales the full scale output of
the ADC. The actual output of the ADC is (UADC,in/UADC,ref)
times the digital full scale output value. Thus, an ADC is capable of
intrinsically normalizing the input signal with respect to the reference
signal and outputting the result digitally as a fraction of the full
scale. So, when applying a position signal 117 (which can be 91) to the
signal input 115 of the analog-to-digital converter and a sum signal 118
(which can be 106 or 113) to the reference input 114 of the
analog-to-digital converter, a system is created that digitally outputs
116 the position value normalized with respect to the sum signal value
(IP/IS or UP/US). The normalization can be performed
within one conversion cycle using parallel architecture (flash)
analog-to-digital converters. Other architectures can also be used but
can introduce a pole in the reference signal. Furthermore, optimal signal
to noise ratios are obtained when the sum value is about the optimal
reference value, which usually is specified by the manufacturer of the
ADC. For two-dimensional position measurements, two analog-to-digital
converters can be used. The sum output of one of the one-dimensional
position detectors can be used to act as input for both analog to digital
converters, or each analog to digital converter can be referenced
mutually by the corresponding direction sum signal of the one-dimensional
position detector. Because in nowadays systems almost all relevant analog
signals are digitized, the normalization method usually requires no extra
components for implementation and in case analog output signals are
required, the digital signal can be made analog by means of a
digital-to-analog converter (DAC), see FIG. 7B. Here the digital output
of the ADC 116 is coupled to the digital input 119 of the DAC. The output
120 of the DAC is an analog signal. Furthermore, digital signal
processing (DSP) and/or recording techniques can be performed on the
digitized position signals weather they are eventually made analog or
not. See FIG. 7c. The normalization method can be applied to any system
in which some signal 115 has to be normalized with respect to another
signal 114. For the digital signal processing, also use of Field
Programmable Gate Array (FPGA) techniques can be made. For analog signal
processing--weather for anti-aliasing filters of other types of
filtering--use of Field Programmable Analog Array (FPAA) techniques can
be made prior to the ADC stage, and/or after the DAC stage.

[0122] It will be apparent to those skilled in the art that various
modifications and variations can be made in the method and system of the
present invention without departing from the spirit or scope of the
invention. The present invention is not limited by the foregoing
description but is intended to cover all modifications, equivalents, and
variations that come within the scope of the spirit of the invention and
the claims that follow.

Patent applications in class Position of detected arrangement relative to projected beam

Patent applications in all subclasses Position of detected arrangement relative to projected beam